Wireless local area networks (WLANs) have been designed for data communication and have found widespread acceptance and proliferation in the industry. Two wireless broadband LANs (WLANs) are standardized in the 5 GHz band, namely IEEE 802.11a and ETSI HIPERLAN/2. The physical layers of both standards are very similar: they both use a modulation technique called “Orthogonal Frequency Division Multiplexing (OFDM)” and can provide up to 8 different transmission modes with data rates ranging from 6 Mbps up to 54 Mbps. This multi-rate capability enables a WLAN station to select a transmission mode which is best appropriate to the current radio channel quality to reach the best performance.
In general, adaptive adjustment of the transmission rate is achieved by having a receiver estimating the channel link quality, deriving from this estimation the rate to be used in future transmissions, and sending this information back to the transmitter. The main issues for an efficient link adaptation mechanism are the determination of the parameters to be used for the link quality estimation, e.g. packet error rate, signal to noise ratio, received signal strength, carrier to interference ratio, etc., how to measure them, and how to select the appropriate rate out of the measurement results.
In HIPERLAN/2, it is the responsibility of an Access Point (AP) to dynamically select any of the available PHY (physical layer) modes for the down- and uplink transmissions. A Mobile Terminal (MT) continuously measures the quality of the downlink and suggests a suitable downlink transmission rate to the AP. For the uplink the AP itself performs the link quality estimation. The standard however does not specify how the link quality estimation and the corresponding transmission mode selection are performed. S. Simoens and D. Bartolomé describe in their article “Optimum performance of link adaptation in HIPERLAN/2 Networks”, VTC 2001, a method for estimating the SNIR (Signal to Noise plus Interference Ratio) and based on this estimation determining the transmission rate that would maximize the throughput of an HIPERLAN/2 network. Similarly, Z. Lin, G. Malmgren, and J. Torsner studied in their article “System Performance Analysis of Link Adaptation in HiperLAN Type 2”, VTC Fall 2000, the performance of the link adaptation of HIPERLAN/2 when using a C/I (Carrier to Interference ratio) as link quality parameter.
The standard IEEE 802.11 only specifies which transmission rates are allowed for which types of MAC (medium-access-control layer) frames, but not how and when to switch between the permitted rates. Furthermore, there is no signaling mechanism specified which would allow a receiver to inform the transmitter about the quality of the communication channel or the rate to be used. The transmitter can change the rate at any time between two consecutive packets, but not in the middle of a sequence of MAC frames belonging to the same packet. The rate at which a MAC frame is transmitted is coded in the header of the physical layer (the so-called PLCP header) which is sent at a fixed rate (6 Mbps in case of IEEE 802.11a) supported by all stations. Thus, after having decoded successfully the PLCP header, the receiver switches to the indicated rate to receive the MAC frame.
Although IEEE 802.11 WLANs are becoming more and more popular, little has been published about the rate adaptation techniques that could be applied to those networks. A. Kamerman and L. Montean describe in “WaveLAN-II: A High-Performance Wireless LAN for the Unlicensed Band”, Bell Labs Technical Journal, Summer 1997, pp. 118-133, a method used in Lucent's WaveLAN-II devices. It is basically an automatic method for switching between two transmission rates, with the high one as the default operating rate. The device switches automatically to the low rate after two consecutive transmission errors and back to the high rate either after ten successful transmissions or after a time out.
As mentioned above, the IEEE 802.11 standard does not specify how rate switching should be executed in case of multi-rate PHY layers. It only specifies which rates have to be used for sending which MAC frames. It even does not provide any protocol means for a receiver to inform the transmitter about the actual link quality or the transmission rate to be used. That is why the link adaptation method described by G. Holland et. al. in “A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks”, ACM/IEEE International Conference on Mobile Computing and Networking (MOBICOM'01) Rome, Italy, July 2001, cannot be applied to current IEEE 802.11 WLANs, since it is based on the principle that the receiver determines the link quality and requests the transmitter to switch to a more appropriate rate.
From the above it follows that there is still a need in the art for an improved and efficient link adaptation method. Moreover, only information available at the transmitter side should be sufficient to guess whether the actual link quality is improving or worsening.